1. Field of the Invention
The present invention relates to an electroless copper plating bath. More particularly, the present invention relates to an electroless copper plating bath including no formaldehyde.
2. Description of the Related Art
Electroless plating is commonly applied as a method for forming an electrode with a uniform film thickness on a substrate having an insulating surface, and on substrates of miscellaneous shapes which have through holes, etc. Particularly, among various types of electroless platings, an electroless copper plating is often utilized when a good electroconductivity is required for the electrode.
An electroless copper plating bath usually contains cupric ions, a cupric ion complexing agent, a reducing agent for reducing the cupric ion to deposit copper and a pH adjusting agent. Furthermore, it contains a stabilizer for improving the stability of the plating bath, a surfactant for improving the properties of the plating film, etc. To briefly explain the mechanism of the electroless copper plating, the reducing agent in the plating bath causes an oxidation reaction with a catalytic action of copper, emitting electrons. Subsequently, the cupric ion is reduced by receiving the emitted electrons, depositing a copper plating film on an article to be plated.
In almost all of the industrially utilized electroless copper plating baths, formalin or an aqueous solution of formaldehyde is used as the reducing agent. However, formalin is not desirable from the viewpoint of the working environment, since it is volatile and has a strong odor. Furthermore, there are arguments that it is carcinogenic. Thus formalin has a number of problems.
In consideration of the above-described problems, it was proposed recently to use glyoxylic acid as a reducing agent substituting for formalin in electroless copper plating. Glyoxylic acid has a structure similar to that of formalin, and therefore it is believed that it has an oxidation reaction mechanism similar to that of formalin in the electroless copper plating reactions. Furthermore, there is a report that a copper plating film obtained from an electroless copper plating bath using glyoxylic acid as the reducing agent has properties equivalent to those of the copper plating film obtained from an electroless copper plating bath using formalin.
However, the oxidation reaction of glyoxylic acid proceeds more slowly than that of formalin, though it can be caused by the catalytic action of copper. That is, glyoxylic acid emits fewer electrons from the oxidation reaction, and therefore the plating reaction proceeds more slowly in the electroless copper plating bath using glyoxylic acid as the reducing agent than in the electroless copper plating bath using formalin. As a result, when a copper electrode for an electronic part is formed using the electroless copper plating bath in which glyoxylic acid is added as the reducing agent, the time required for the plating is longer and the production efficiency is lowered. This poses a problem.
Accordingly, it is an object of the present invention to provide an electroless copper plating bath which does not use the formaldehyde which is not desirable as the reducing agent from the viewpoint of working environment, and which provides a plating reaction velocity on the same level as the plating bath in which formaldehyde is used as the reducing agent.
As a result of studies that had been pursued zealously to solve the above-mentioned problems, it was found through electrochemical polarization measurements that the oxidation reaction of glyoxylic acid with the aid of the catalytic action of copper is accelerated by adding a carboxylic acid to the electroless copper plating bath. Although the details of the reaction mechanism has not been clarified, it is believed as follows: glyoxylic acid tends to be more susceptible to oxidation as the pH is raised; and accordingly, when a carboxylic acid is added to the plating bath, it prevents the pH of the plating bath from decreasing in the vicinity of the interface between copper and glyoxylic acid with the result that the glyoxylic acid becomes more susceptible to the catalytic action of copper, thus emitting electrons more easily.
Therefore, one aspect of the present invention is an electroless copper plating bath containing a cupric compound, a cupric ion complexing agent, a reducing agent, and a pH adjusting agent, in which a carboxylic acid is added as a reaction accelerator to accelerate the oxidation reaction of the above-described reducing agent. The present invention is preferably applied when the above-described reducing agent is a reducing agent other than formaldehyde. It is more preferably applied when the reducing agent is glyoxylic acid.
As explained above, even though a reducing agent such as glyoxylic acid that gives a slower oxidation reaction than formaldehyde is used as a reducing agent for electroless copper plating, a plating velocity on the same level as that in the case in which formaldehyde is used can be obtained by further adding a carboxylic acid.
As a favorable carboxylic acid to be added, enumerated are a monocarboxylic acid selected from the group consisting of glycolic acid, acetic acid and glycine, a dicarboxylic acid selected from the group consisting of oxalic acid, succinic acid, malic acid and malonic acid, as well as a tricarboxylic acid selected from the group consisting of citric acid and nitrilotriacetic acid.
It is preferable that the concentration of the carboxylic acid added to the plating bath is from about 0.01 to 0.2 mol/L. The electroless copper plating can be performed on the surface of a substrate by immersing the substrate into such an electroless copper plating bath as described above, and therefore an electronic part having a substrate with a copper electrode formed thereon can be formed by using the electroless copper plating bath according to the present invention. Furthermore, the present invention can be applied especially favorably when a substrate formed from a ceramic material is subjected to the electroless copper plating.
The electroless copper plating bath according to the present invention comprises a cupric compound, a cupric ion complexing agent, a reducing agent, a pH adjusting agent and a reaction accelerator to accelerate the reaction of the reducing agent. A surfactant and a stabilizer for improving the stability of the plating bath are also added, as necessary.
Any cupric compound may be used as long as it can supply a cupric ion. Copper (II) sulfate, copper (II) chloride, copper (II) nitrate, etc. can be used, for example. Among them, copper sulfate is the most favorable. By adding such a cupric compound to the plating bath, the cupric ion concentration in the plating bath is adjusted to a value preferably in the range of from about 0.001 to 0.2 mol/L, and more preferably in the range of from about 0.01 to 0.05 mol/L.
As the cupric ion complexing agent, enumerated are Rochelle salt which makes a strongly bonded complex with a cupric ion, ethylenediaminetetraacetic acid, diethylenetriaminepentaacetic acid, etc. Among these complexing agents, ethylenediaminetetraacetic acid and its derivatives are particularly preferable. The long-term stability and the plating film depositing velocity of the plating bath are especially good when such a complexing agent is added. It is noted that the concentration of the complexing agent in the plating bath is adjusted to a value about two to ten times larger than the cupric ion concentration.
When glyoxylic acid is used as the reducing agent, the concentration in the plating bath is adjusted to a value preferably in the range of from about 0.01 to 0.5 mol/L, and more preferably in the range of from about 0.03 to 0.2 mol/L.
As the reaction accelerator, a carboxylic acid is used. A monocarboxylic acid such as glycolic acid, acetic acid or glycine, a dicarboxylic acid such as oxalic acid, succinic acid, malic acid or malonic acid, and a tricarboxylic acid such as citric acid or nitrilotriacetic acid, can be used. Among these carboxylic acids, glycolic acid, succinic acid, and citric acid are especially preferable. The concentration of such a carboxylic acid in the plating bath is also adjusted to a value preferably in the range of from about 0.005 to 0.3 mol/L, and more preferably in the range of from about 0.05 to 0.2 mol/L.
As the stabilizer, a nitrogen-containing compound such as 2,2xe2x80x2-dipyridyl, 1,2,4-benzotriazole, 1,10-phenanthroline, or potassium ferrocyanide, a sulfur-containing compound such as thiourea, thiosulfuric acid or 2-mercaptobenzothiazole, and an iodine-containing compound such as sodium iodide or potassium iodide, can be used.
As the pH adjusting agent, sodium hydroxide, potassium hydroxide, lithium hydroxide, sulfuric acid, hydrochloric acid, etc., can be used. The pH of the plating bath is adjusted to a value preferably in the range of from about 11 to 13.5, and more preferably in the range of from about 12 to 12.7.
Furthermore, a nonionic surfactant, an anionic surfactant, a cationic surfactant or an amphoteric surfactant may be added, as appropriate, for improving the properties of the plating film. The amount of the added surfactant is preferably in the range of from about 0.05 to about 5 g/L.